1. Two-Dimensional Arrays and Nested Loops – part 6
Barb Ericson
Georgia Institute of Technology
August 2005
Georgia Institute of Technology

2. Georgia Institute of Technology
Learning Goals
Understand at a conceptual and practical level
How can you make a picture smaller
Scale it down
How can you make a picture bigger
Scale it up
How do you create a collage?
Georgia Institute of Technology

3. Georgia Institute of Technology
Scaling
You can make a picture smaller
Faster to download on the web
Increment the source x and y by a number larger than 1
Don’t use all the source pixels in target
You can make a picture larger
Show more detail
Copy the same source x and y to more than one target x and y
Use source pixels more than once in target
Georgia Institute of Technology

4. Georgia Institute of Technology
Scaling Down a Picture
passionFlower.jpg is 640pixels wide and 480 pixels high
If we copy every other pixel we will have a new picture with width (640 / 2 = 320) and height (480 / 2 = 240) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 2 8 10
Georgia Institute of Technology

5. Scaling Down Algorithm
Create the target picture
Invoke the method on the target picture
Create the source picture
Loop with source x starting at 0 and target x starting at 0 as long as < source width
Increment the source x by 2 each time through the loop, increment the target x by 1
Loop with source y starting at 0 and target y starting at 0 as long as < source height
Increment the source y by 2 each time through the loop, increment the target y by 1
Copy the color from the source to target pixel
Georgia Institute of Technology

6. Georgia Institute of Technology
Scaling Down Method
public void copyFlowerSmaller() {
Picture flowerPicture =
new Picture(
FileChooser.getMediaPath(“passionFlower.jpg"));
Pixel sourcePixel = null;
Pixel targetPixel = null;
// loop through the columns
for (int sourceX = 0, targetX=0;
sourceX < flowerPicture.getWidth();
sourceX+=2, targetX++)
Georgia Institute of Technology

7. Scaling Down Method - Continued
// loop through the rows
for (int sourceY=0, targetY=0;
sourceY < flowerPicture.getHeight();
sourceY+=2, targetY++) {
sourcePixel =
flowerPicture.getPixel(sourceX,sourceY);
targetPixel = this.getPixel(targetX,targetY);
targetPixel.setColor(sourcePixel.getColor()); }
sourceY+= 2 means the same as sourceY = sourceY + 2;
Georgia Institute of Technology

8. Trying Copy Flower Smaller
Create a new picture half the size of the original picture (+ 1 if odd size)
Picture p1 = new Picture(320,240);
Copy the flower to the new picture
p1.copyFlowerSmaller();
Show the result
p1.show();
Georgia Institute of Technology

9. Thinking Through Scaling Up
Copy each pixel in the source multiple times to the target
Source (0,0) Target (0,0)
Source (0,0) Target(1,0)
Source (1,0) Target(2,0)
Source (1,0) Target(3,0)
Source (2,0) Target(4,0)
Source (2,0) Target(5,0)
Source (0,0) Target(0,1)
Source (0,0) Target(1,1) 1 2 1 2 3 4 5 6 1 1 2 3 4 5 1 2 3 4 5 6 1
What could we add to source x and source y so that it is 0 twice, then 1 twice, then 2 twice, etc.
Remember that 0.5 is 0 when you cast to integer. 2 3
Georgia Institute of Technology

10. Georgia Institute of Technology
Scaling Up Algorithm
Create the target picture
Invoke the method on the target picture
Create the source picture
Loop with source x starting at 0 and target x starting at 0 as long as < source width
Increment the source x by 0.5 each time through the loop, increment the target x by 1
Loop with source y starting at 0 and target y starting at 0 as long as < source height
Increment the source y by 0.5 each time through the loop, increment the target y by 1
Copy the color from the source to target pixel
The source x and y will need to be double variables. Cast them to int to lose the fractional part to get the pixel x and y.
Georgia Institute of Technology

11. Georgia Institute of Technology
Scaling Up Exercise
Write a method copyFlowerBigger to scale up the picture flower1.jpg when you copy it to 640x480.jpg
Save the result to a file using
pictureObj.write(“file”);
Georgia Institute of Technology

12. Georgia Institute of Technology
Create a Collage
One of the things that you can do with pictures is create a collage
There are two pictures of flowers
flower1.jpg
flower2.jpg
Both pictures are 100 pixels wide
The target picture is created from file 640x480.jpg
The Picture’s bottom left is at x = 0 y = height - 5
Georgia Institute of Technology

13. Create Collage Algorithm
Create the target picture object
Using the 640x480 file
Invoke the method on the target picture
Create the flower picture objects
using flower1.jpg as source1Picture
using flower2.jpg as source2Picture
Set targetBottomY to the targetPicture height – 5
5 pixels from bottom of picture
Georgia Institute of Technology

14. Create Collage Algorithm - Cont
Copy all of source1Picture to the current picture starting at x=0, y=targetBottomY – source1Picture’s height
Copy all of source2Picture to the current picture starting at x=100, y = targetBottomY – source2Picture’s height
Negate source1Picture
Copy all of source1Picture to the current picture starting at x=200, y= targetBottomY – source1Picture’s height
Clear the blue from source2Picture
Copy all of source2Picture to the current picture starting at x=300, y= targetBottomY – source2Picture’s height
Copy all of source1Picture to the current picture starting at x=400, y=targetBottomY – source1Picture’s height
Georgia Institute of Technology

15. Create Flower Collage Method
public void createFlowerCollage() {
Picture source1Picture =
new Picture(FileChooser.getMediaPath("flower1.jpg"));
Picture source2Picture =
new Picture(FileChooser.getMediaPath("flower2.jpg"));
int targetBottomY = this.getHeight() - 5;
// copy source1Picture to 0, targetBottomY - height
this.copy(source1Picture,0,0,
source1Picture.getWidth(),
source1Picture.getHeight(),
0,targetBottomY -
source1Picture.getHeight());
// copy source2Picture to 100, targetBottomY - height
this.copy(source2Picture,0,0,
source2Picture.getWidth(),
source2Picture.getHeight(),
100,targetBottomY -
source2Picture.getHeight());
Georgia Institute of Technology

16. Create Flower Collage Method - Cont
// negate the source1Picture
source1Picture.negate();
// copy negated source1Picture to 200
this.copy(source1Picture,0,0,
source1Picture.getWidth(),
source1Picture.getHeight(),
200,targetBottomY -
source1Picture.getHeight());
// clear the blue from source 2 picture
source2Picture.clearBlue();
// copy source2Picture to 300
this.copy(source2Picture,0,0,
source2Picture.getWidth(),
source2Picture.getHeight(),
300,targetBottomY -
source2Picture.getHeight());
// copy negated source1Picture to 400
400,targetBottomY - }
Georgia Institute of Technology

17. Testing createFlowerCollage
String file = FileChooser.getMediaPath(“640x480.jpg”);
Picture p = new Picture(file);
p.show();
p.createFlowerCollage();
p.repaint();
Georgia Institute of Technology

18. Create Collage Exercise
Try creating a collage
At least 4 copies of an image in it
The original image and 3 changes to the original image
Scale, rotate, crop, change colors
Then mirror the whole picture horizontally
Save your collage using
pictureObj.write(fileName);
See for some examples from Georgia Tech students.
Georgia Institute of Technology

19. Georgia Institute of Technology
Summary
You can scale a picture down
By not copying all the pixels
Increment the source index by a number > 1
You can scale a picture up
By copying the source pixels more than one time
You can increment the source index by 0.5 and then cast it to integer
You can create an image collage
Using the methods we have written
Georgia Institute of Technology